A system has been developed to permit customer loops served by a digital loop carrier (DLC) to be tested from a loop maintenance operating system (LMOS) or a repair service bureau (RSB). This system provides a means of switching that will connect incoming RSB test trunks to a DC bypass around a carrier system to a distribution pair terminating on a remote terminal (RT) or, in the absence of a DC bypass pair (Fiber Optic DLC), allows distribution pairs to be tested by a Remote Test Unit (RTU) in the RT. The method requires an item of central office equipment, called a pair gain test controller (PGTC), which will support all carrier systems deployed from a particular wire center. The PGTC is a microcomputer-controlled device that interacts with circuitry in each of a plurality of carrier systems to provide metallic access to customer drops. This allows the LMOS or RSB to perform standard tests on a drop, while at the same time, the PGTC performs a series of automatic transmission and signaling tests on the derived carrier channel. A description of the PGTC in connection with channel and drop testing is described in detail at pages 2373-2385 of the AT&T Bell Laboratories Technical Journal, Vol. 63, No. 10, Part 2, of December 1984. The channel testing features are described in further detail at pages 2315-2318.
Many systems have proprietary test schemes, usually using a test head at the central office (CO) and a test unit at the remote terminal (RT) coordinated by an operations system (OS).
The testing process consists of two distinct intervals: the test set-up interval, where the set-up and cut-through of the DC bypass path from the RSB to the customer are established or where a Test System Controller to RTU communication link is established in Fiber Optic DLC applications; and the testing interval, where the PGTC conducts a test of the derived channel while the RSB or RTU is testing the customer's drop. The test set-up interval is initiated by first dialing the customer's telephone number over one of the RSB test trunks. A handshaking sequence of events at the interface between the PGTC and the test unit and channel unit under test then proceeds during the test set-up interval which takes little more than approximately one second.
During the testing interval, and while the RSB test facility is performing its normal tests of the customer's drop, the PGTC conducts an automatic transmission and signaling test of the derived channel. This test is conducted immediately after cut-through of the DC test path and is completed within approximately two seconds for single parties. The results of these tests are forwarded as a tone or sequence of tones to the RSB test facility. The transmission tests check that transmission parameters of the carrier channel are not grossly out of limits. The signaling checks insure that the carrier system channel can detect and repeat the necessary telephone control and supervision signals. The transmission and signaling tests are described at page 2378 of the above cited technical chart in section 3.2.2.
The transmission tests require that the carrier system RT be able to apply three types of terminations to the RT channel unit under test: a reflective termination (short circuit), an absorptive termination (900 ohms) and a resistance to ground that is less than 3,000 ohms. The resistance to ground is poled through diodes to allow current to flow from either a positive or negative battery, depending on the test being conducted.
Two types of return loss measurements are performed: (1) a measure of round trip channel loss performed with the RT channel unit terminated by a reflective termination and a (2) a measure of echo-return loss with the RT channel unit terminated by an absorptive termination. A round trip idle channel noise test is also made by turning off a swept oscillator in the PGTC channel tester unit while the RT channel unit has an AC reflective termination.
The automatic sequences are performed in a manner that excerises all of the signaling states while conducting the transmission measurements. The RT channel test unit is capable of recognizing various signaling tests and responds by applying an appropriate termination across tip and ring of the RT channel unit. These terminations are detectable at the intermediate central office terminal by the PGTC and each allows a particular transmission measurement to be made. A state diagram for the sequencing of the RT terminations is shown in FIG. 7 of the above cited technical journal at page 2381. The state diagram shows an echo-return loss test conducted with an absorptive termination, a round-trip channel loss test is shown with a reflective termination tested with an oscillator at the PGTC tester unit and an idle channel noise test may be made with the oscillator off. At this point, the channel has been checked for transmission parameters and its signaling has been verified as sufficient to support a single party service.
The RSB test system can obtain the results of the automatic tests from the PGTC which presents the results as a pattern of audible tones and as DC voltages applied to the test trunk as summarized in table 2 on page 2385 of the above cited technical journal. When the RSB test system has completed all the tests of the customer's drop and has received the results of the PGTC tests, it must issue a disconnect signal to the PGTC which tells the PGTC to release the test connection. The PGTC, in turn, notifies the carrier system to restore all connections to normal.
MLT is typically run from LMOS or RSB on a routine basis overnight. Craft then gets a list of bad lines to fix. Craft can also call LMOS to do specific tests on a specific line to see if maintenance actions performed have fixed the problem. But the craft person cannot personally run any of these tests by himself. Furthermore, many independent telephone companies have neither craft test capability nor MLT and therefore must resort to manual test methods.
Unfortunately, it is particularly uneconomical for some operating companies (especially independents) to use mechanized loop testing (MLT), although they would very much like some kind of loop test capability and/or line card diagnostics.